1. Field of the Invention
The present invention relates to a wireless local area network (WLAN) device that can receive signals of multiple modulation types. In particular, the present invention relates to a voting block that quickly and accurately identifies the modulation type of the signal.
2. Description of the Related Art
Wireless local area networks (WLANs) are becoming increasingly popular as communication networks. The IEEE 802.11 standards provide guidelines for the operation of devices operating in WLANs. Specifically, the 802.11a standard defines communication in the 5 GHz band using data rates of 6, 12, 18, 24, 36, and 54 Mbps. The 802.11b standard defines communication in the 2.4 GHz band using data rates of 1, 2, 5.5, and 11 Mbps. A recently ratified addition to the 802.11 standards, called the 802.11g standard, uses the high data rates of 802.11a in the frequency band of 802.11b, i.e. 2.4 GHz.
The 802.11g standard envisions that 802.11b devices can communicate with 802.11g devices that operate in the same WLAN. Note that as used herein, the term “device” can refer to either an access point or a client.
To ensure backwards compatibility with 802.11b devices while allowing data rate increases, the 802.11g standard incorporates the modulation schemes from both the 802.11a and 802.11b standards. In general, modulation refers to the addition of information to a carrier. Typical modulation techniques include frequency modulation wherein the frequency of the carrier waveform is varied.
The 802.11b standard uses a modulation scheme called complementary code keying (CCK) (note that 802.11b can also use Barker code chipping at the 1 and 2 Mbps rates) whereas the 802.11a standard uses another modulation scheme called orthogonal frequency division multiplexing (OFDM). The OFDM scheme involves encoding information upon a plurality of sub-carriers, which are then modulated onto a carrier signal. The proposed 802.11g standard can use other modulation schemes in addition to CCK and OFDM. For example, a modified OFDM scheme called Extended Range (XR), which has been developed by Atheros Communications, Inc. to operate concurrently with the 802.11a, 802.11b, and 802.11g standards, can also be used.
Of importance, the 802.11g standard provides no implementation requirements as to how a modulation type should be identified. Therefore, a need arises for a technique that quickly and accurately identifies signals of different modulations when received by the WLAN device.